The present invention concerns a drive for the driven drafting arrangement rolls of long spinning machines extending over the full length or over considerable parts of the length of the machine, preferably of ring spinning machines, in which the drafting arrangement rolls arranged on the same longitudinal side of the machine and defining a main drafting zone for the fibre slivers to be drafted, are mutually connected at one of their ends by first slippage-free transmission elements which effect the ratio of rotational speeds of these rolls determining the draft ratio.
With textile machines of this type there is a tendency, active since the very origins, to increase the number of working positions, e.g. of the spindles in the case of the ring spinning machine, driven by one single drive headstock. This tendency is prompted mainly by economic factors, as the increase in the number of working positions per machine results in a reduction of the price per working position, in a reduction of space per working position and in most cases, also in operating advantages.
One difficulty in achieving the above mentioned increase, however, is presented by the bottom rolls of the drafting arrangements of such machines, as in excessively long rolls the deformations caused by torsion can result in unacceptable working conditions. This concerns predominantly the rolls defining the main drafting zone of the drafting arrangement, i.e. e.g. the middle roll and the delivery roll of the double apron drafting arrangement, as widely used today on various spinning machines, preferentially on ring spinning machines, but also e.g. on the roving frame. In a main drafting zone of such type, the fibre mass is drafted to its final fineness before twist is imparted at a draft ratio of 10 fold and more, normally of about 30 fold. Any distortion, however small, of one roll active in the main drafting zone of a drafting arrangement with respect to the other roll immediately causes a considerable drafting defect resulting in a thick place and in a thin place in the drafted yarn. In many cases then very undesirable yarn breakages result. Experience with especially long spinning machines has shown, that the danger of distortion of the roll of the drafting arrangement is particularly great for the middle roll of a double apron drafting arrangement. Onto this roll, which as a rule also acts as a deflecting roll for the bottom apron of the double apron drafting arrangement, particularly strong braking forces act tangentially, which considerably exceed similar forces acting on the delivery rolls.
This difference is due to the rotational speed, which is reduced with respect to the delivery roll by the draft ratio, and due to the higher friction forces mainly generated by the apron which acts on this roll, and to the strong drafting forces acting on this roll in the predrafting zone. Due to these higher braking forces the middle drafting roll during the operation of the drafting arrangement thus is distorted to a higher degree than the delivery roll. If now the spinning machine is stopped, the forces acting tangentially upon the roll, and particularly the important drafting forces, are released to a large extent, in such manner that the input roll of the main draft zone, which is more distorted with respect to the delivery roll of the main drafting zone in the backward sense, tends to reverse this distortion and distort back in the normal sense of rotation. As the machine is thereafter started up, drafting defects generated in this manner already cause yarn breakages.
The situation described here for the drafting arrangement of the conventional spinning machine is further exacerbated in that during the subsequent start-up of the machine higher static friction forces are to be overcome by the input roll of the main drafting zone than by the delivery roll, which forces cause progressive distortion from the beginning of the input roll to its end. Also due to this distortion, the degree of which exceeds the distortion during normal operation, drafting defects and thus yarn breakages arise.
A reduction of the distortion mentioned by increasing the diameter of the rolls is excluded because of spinning technology requirements as an increase in diameter could be achieved only to the detriment of the quality of the fibre control during the drafting process.
Thus, it has been proposed already to avoid the above mentioned disadvantage of a long spinning machine by either driving the drafting rolls separately at both ends, in which arrangement the drafting rolls can be divided all the way or at the middle of the machine, as e.g. shown in U.S. Pat. No. 3,339,361 or by interconnecting the rolls at least in pairs via separate toothed gear drives, as shown in German DE-OS No. 26 41 434. All these proposed arrangements are suitable for avoiding the undesirable distortion of the rolls; they show, however, substantial disadvantages. Thus, they require a complete and exactly synchronous double drive for the drafting rolls. This solution is expensive, and still more important, it implies, at least in an arrangement using continuous rolls, a very dangerous source of errors. If the draft ratio in the main drafting zone, which is to be adapted by correspondingly choosing gears (draft change gears) or similar elements, due to operator's error is not set to the same value at both side, torsion-forced breakage of one of the rolls inevitably occurs. This solution thus requires highest attention of the operating personnel and thus runs contrary to the intentions of the spinning mills of facilitating easy and error-free operation of the spinning machine.